The present invention relates to assemblies comprising a vacuum servomotor and a master cylinder used on motor vehicles to assist the control of braking.
Such a servomotor conventionally has, in an outer casing, a space divided in leakproof fashion into a front chamber and a rear chamber by a movable partition, at least one first resilient means tending to displace the movable partition towards the rear chamber, the front chamber being permanently subjected to a vaccum, a pressure difference being established between the front and rear chambers by a valve means capable of connecting the rear chamber to the atmosphere when the brake pedal is actuated. This pressure difference is used to displace the movable partition towards the front chamber counter to the resilient means and to actuate the pistons of a brake master cylinder. To ensure appropriate metering of the effort on the pedal, the conventional servomotors have a reaction disc in the valve means, such as that described in the document FR-A-2 630 694.
It has been proposed to replace the reaction disc, which is a wearing part, by another reaction means, as in the document FR-A-2 493 783, where the reaction means is formed by a piston controlled by the partial sum of the hydraulic pressures prevailing in the chambers of the master cylinder.
This latter system does not permit control of the development of the reaction in the course of the increase in the effort of the servomotor on the master cylinder, which is desired by motor-vehicle manufacturers, so that the pressure generated by the master cylinder does not vary as a linear function of the effort exerted on the pedal.